Hydrogen represents a next generation energy carrier that can efficiently produce electricity from its chemical energy. Because of the depletion of fossil fuel reserves and the increasing CO2 emissions, hydrogen is a promising power source for mobile and stationary applications. To fully commercialize proton exchange membrane (PEM) fuel cells, sustainable hydrogen production is indispensable. The conversion of water to hydrogen and oxygen (2H2O(1)→2H2(g)+O2(g) (electrolysis) using carbon emission-free energy sources of electricity such as solar and wind power is a sustainable energy storage approach. Efficient hydrogen evolution reaction (HER) in acidic condition (2H++2e−→H2) remains difficult without the use of platinum catalysts.
The U. S. Department of Energy (DOE) has set a cost goal for hydrogen at $2.00-3.00/kg, including production, delivery and dispensing, a level at which the DOE estimates that hydrogen will be cost competitive with petroleum fuels (Turner et al. (2008) International J. Energy Res. 32:379). According to a 2012 DOE report (Ayers (2012) DOE annual merit review proceedings, Hydrogen and Fuel Cells Program, Arlington, Va.), a large part of the premium on an industrial proton-exchange-membrane electrolyzer is the cost of the “membrane-electrode-assembly,” and half of that cost is due to platinum. State-of-the-art hydrogen production costs about $4-5/kg based on Pt-catalyzed devices. The high cost of platinum ($1450/oz. in August 2012) and its scarcity makes its use an impediment to large-scale commercial application of proton-exchange-membrane electrolyzers.
A general approach for overcoming this obstacle is to reduce the use of noble metals or replace them with low cost non-precious catalysts. Previous Pt alternatives encountered barriers such as high activating voltage (Helm, et al. (2011) Science 333: 863; and Harnisch et al. (2009) App. Catalysis B: Environmental 89:455), low surface area (Merki et al. (2012) Chem. Sci. 3:2515), intrinsic resistance (Chen et al. (2011) Nano Letters 11:4168), or difficulties relating to production and scale up (LeGoff et al. (2009) Science 326:1384).
Molybdenum, one of the most abundant transition metals, contributes high corrosion resistance to stainless steel but has generally proven to be unsatisfactory in electrocatalytic activity towards hydrogen evolution. Accordingly, there has been an ongoing effort to find affordable hydrogen production routes based on molybdenum for water electrolysis (Karunadasa et al. (2012) Science 335: 698; Vrubel et al (2012) Energy. Environ. Sci. 5:6136; Chen et al. (2012) Ang. Chemie Int'nl. Ed. 51:6131), as well as for photoelectrochemical water splitting (McKone et al. (2011) Energy Environ. Sci. 4:3573; Seger et al. (2012) Ang. Chemie Int'nl. Ed. 51:9128).
Hence, there exists a bong standing need to provide tow-cost transition metal catalysts using simple environmentally friendly approaches.